An electromagnetic force-compensation direct-measuring system, hereinafter referred to as direct-measuring system, is distinguished by the fact that the load receiver is connected through a force-transmitting rod directly to the force compensation device.
Under the principle of electromagnetic force compensation, the force that is caused by a load resting on the weighing pan or on the load receiver is counterbalanced by a force-compensation device consisting of at least one permanent magnet and a coil, wherein the current is being measured which flows through the coil in order to generate the compensating force. The value determined by this measurement is proportional to the force placed on the weighing pan. However, the measurement value also depends on the position of the coil in the magnetic field of the permanent magnet, and the coil therefore always has to be in the same position relative to the magnet when taking a measurement. The position of the coil after the load has been applied is measured by way of a position sensor, and the current in the coil is increased until the position change of the coil relative to the permanent magnet which occurred as a result of the load has been compensated. At this point the coil current is measured which, in turn, represents a measure for the weight of the load resting on the weighing pan.
A direct-measuring system is disclosed in CH 593 481 A5, wherein the load receiver is coupled directly to the force-compensation device by way of a force-transmitting rod. Attached to the force-transmitting rod is the moving part of the position sensor, while the stationary part of the position sensor is rigidly connected to the portion of the weighing cell that is in solid connection with the housing or, more specifically, to the stationary portion of the force-compensation device.
This direct-measuring system is used with preference in the range of small weighing loads. The precision of the measurement depends in essence on the resolution and on the arrangement of the position sensor in the direct-measuring system. The load receiver and the coil of the force-compensation device have to be precisely guided in their mobility relative to the stationary part of the weighing cell. This is accomplished through a parallel-guiding mechanism whose movable parallel leg is connected to the force-transmitting rod and whose stationary parallel leg is rigidly connected to the housing-mounted portion of the weighing cell. The movable parallel leg and the stationary parallel leg are connected to each other by two parallel guides with thin, flexibly bending pivot areas, so-called flexure pivots. However, it is also possible to use elastically flexible parallel guides, in which case the flexure pivots are omitted. When a load is placed on the load receiver, the force-transmitting rod moves in the direction of the load, causing the parallel guides to be deflected and the flexure pivots or elastically flexible parallel guides to be bent.
The parallel-guiding mechanism normally has a position-restoring force as indicated by its spring constant which, like a load placed on the load receiver, has an effect on the displacement of the coil and should likewise be compensated.
Normally, the coil of the force-compensation device is electrically connected through thin wires to a control circuit. This arrangement has the disadvantage that in addition to providing the electrical connection, the wires also establish a mechanical connection from the stationary to the movable part of the weighing cell. This introduces an additional spring constant into the direct-measuring system, which interacts with the parallel-guiding mechanism and can introduce errors into the weighing result. The wires are normally connected by soldering and are configured as exceptionally thin filaments in order to keep the added spring constant as small as possible. However, the wire filaments are difficult to connect and it can easily happen that one of the wires breaks loose, making the balance inoperable.
The spring constant which results from the mechanical connection of the movable and the stationary part of the weighing cell through the coil circuit connections affects most of all the performance of weighing cells for the range of small weighing loads, and/or in case of a high resolution it affects the weighing result, because in this case even the smallest changes of the spring constants are enough to cause a change of the measurement result.
It has further been found in particular in compact weighing modules with direct-measuring systems for multi-module weighing devices of the type disclosed for example in EP 1726926 A1, that the drift of the zero point of a direct-measuring system is negatively affected if the parts of the parallel-guiding mechanism and/or of the force-transmitting rod that are connected to the position sensor exhibit a different amount of thermal expansion.